Components of First-Trimester Combined Screening

The first-trimester combined screening test is performed between 11 and 13+6 weeks of gestation to assess the risk of fetal chromosomal abnormalities such as Down syndrome (trisomy 21), trisomy 18, and trisomy 13. It combines a maternal age-based risk with two maternal serum biochemical markers and one ultrasound measurement.

The correct combination is β-hCG, Nuchal translucency, and PAPP-A, corresponding to Option 1 (1, 3 and 4 only).

Analyzing Each Component

• 1. β-hCG (free beta-human chorionic gonadotropin): A maternal serum marker measured in the first trimester. In pregnancies affected by Down syndrome, free β-hCG levels tend to be elevated. This is a core component of first-trimester screening — correctly included.
• 2. MS AFP (Maternal Serum Alpha-Fetoprotein): AFP is a second-trimester marker, used as part of the "double", "triple", or "quadruple" marker tests performed between 15 and 20 weeks. It is not measured or used in first-trimester combined screening — correctly excluded.
• 3. Nuchal translucency (NT): An ultrasound measurement of the fluid-filled space at the back of the fetal neck, performed between 11 and 13+6 weeks. Increased NT thickness is associated with a higher risk of chromosomal abnormalities and cardiac defects. This is an essential ultrasound component of the first-trimester screen — correctly included.
• 4. PAPP-A (Pregnancy-Associated Plasma Protein-A): A placental glycoprotein measured in maternal serum during the first trimester. Levels are typically decreased in Down syndrome pregnancies. It is one of the two biochemical markers (along with β-hCG) used in first-trimester combined screening — correctly included.

Why the Other Options Are Incorrect

• Option 2 (1, 2 and 3 only): Incorrectly includes MS AFP, which belongs to second-trimester screening, and wrongly omits PAPP-A, one of the two essential first-trimester biochemical markers.
• Option 3 (1, 2, 3 and 4): Incorrectly includes MS AFP alongside the correct three components; AFP has no role in the first-trimester combined test.
• Option 4 (2 and 4 only): Incorrectly includes MS AFP and omits both β-hCG and nuchal translucency, which are indispensable parts of the first-trimester combined screen.

Conclusion

The first-trimester combined screening test consists of β-hCG, PAPP-A, and nuchal translucency (items 1, 3, and 4). MS AFP is reserved for second-trimester screening and is not part of this combination, making Option 1 the correct answer.